Spinodal copper-based alloys, e.g., spinodal copper-nickel-tin alloys have recently been developed as commercially viable substitutes for copper-beryllium and phosphor-bronze alloys which are currently prevalent in the manufacture of shaped articles such as electric wire, springs, connectors and relay elements. The equilibrium composition of these spinodal alloys is characterized in that such alloys are in a single phase state at temperatures near the melting point of the alloy but in a two-phase state at room temperature; the spinodal structure is characterized in that, at room temperature, the second phase is finely dispersed homogeneously throughout the first phase rather than being situated at the first phase grain boundaries. Among the alloy properties on which the aforementioned uses, as well as other uses, are based are: high strength; good formability; corrosion resistance; solderability and electrical conductivity. Spinodal Cu-Ni-Sn alloys exhibiting desirable combinations of properties are disclosed in U.S. Pat. No. 3,937,638; U.S. Pat. No. 4,052,204, reissued as U.S. Pat. No. Re. 31,180; U.S. Pat. No. 4,090,890; and U.S. Pat. No. 4,260,432, all in the name of J. T. Plewes.
U.S. Pat. No. 3,937,638 discloses a treatment of a Cu-Ni-Sn cast ingot which involves homogenizing, cold working, and aging which leads to a predominately spinodal structure in the treated alloy. For example, in the case of an alloy containing 7% Ni, 8% Sn and the remainder copper, an exemplary method calls for homogenizing the cast ingot, cold working to achieve 99% area reduction and aging for 8 seconds at a temperature of 425.degree. C. The resulting article has a 0.01% yield strength of 173,000 psi and ductility of 47% area reduction to fracture.
U.S. Pat. No. 4,052,204 discloses quaternary alloys containing not only Cu-Ni-Sn but also at least one additional element selected from among the group consisting of Fe, Zn, Mn, Zr, Nb, Cr, Al and Mg. A predominantly spinodal structure is produced in these alloys by treatment of homogenizing, cold working and aging analogous to the treatment disclosed in U.S. Pat. No. 3,937,638.
U.S. Pat. No. 4,090,890, discloses cold rolled and aged strip material made of alloys having a composition similar to the composition of alloys disclosed in U.S. Pat. No. 3,937,638 and U.S. Pat. No. 4,052,204 and having not only high strength, but also essentially isotropic formability. As a consequence, such strip material is particularly suited for the manufacture of articles which require bending of the strip in directions having a substantial component perpendicular to the rolling direction.
U.S. Pat. No. 4,260,432 discloses Cu-Ni-Sn alloys further containing specified quantities of at least one member of the group consisting of Mo, Nb, Ta, V and Fe which is treated by a short time, low temperature anneal followed by a rapid quench, cold working (optional) with least 25% area reduction and aging. Since the alloys disclosed in this patent do not require cold deformation, such alloys are also suited for the manufacture of articles by hot working as well as cold working, casting, forging, extruding or hot pressing. The resulting articles are said to be strong, ductile and have isotropic formability.
Cu-Ni-Sn alloys and their properties are also a subject of the following papers: L. H. Schwartz, S. Mahajan and J. T. Plewes, "Spinodal Decomposition In A Cu-9 wt % Ni-6wt % Sn Alloy", "Acta Metallurgica, Vol. 22, May 1974, pp. 601-609; L. H. Schwartz and J. T. Plewes, "Spinodal Decomposition in Cu-9 wt % Ni-6wt % Sn-II. A Critical Examination of the Mechanical Strength of Spinodal Alloys", Acta Metallurgica, Vol. 22, July 1974, pp. 911-921; John T. Plewes, "Spinodal Cu-Ni-Sn Alloys are Strong and Superductile", Metal Progress, July 1974, pp. 46-50; J. T. Plewes, "High-Strength Cu-Ni-Sn Alloys by Thermomechanical Processing", Metallurgical Transactions A, Vol. 6A, March 1975, pp. 537-544.
Additionally, copper-based alloys containing Ni and Sn having good strength and bend properties is an object of a method disclosed in U.S. Pat. No. 3,941,620, issued to M. J. Pryor et al. Pryor et al discloses a method for treating an ingot by homogenizing, cold rolling, aging and again cold rolling. After aging, the sample is cooled slowly as opposed to being quenched.
This earlier work in the Cu-Ni-Sn system identified the occurrence, over a broad compositional regime, of two competing reactions during a low temperature aging sequence. The first reaction is the formation of the equilibrium (.alpha.+.gamma.) phase which nucleates discontinuously at the grain boundaries. There is a definite incubation time for this reaction to occur which is a function of cold work, aging temperature, aging time and composition. The second reaction is a continuous demiscibility process, termed spinodal decomposition, which occurs homogenously throughout the matrix. There is no incubation period for this process. Generally, the nucleation and subsequent growth of the .alpha.+.gamma. phase occurs early in the spinodal transformation sequence. Since this reaction occurs initially at grain boundaries, the alloys are rendered brittle. It was later discovered as shown in the aforementioned patents, that a process which includes a cold working step with a high degree of cold work prior to the final low temperature aging sequence dramatically accelerates the kinetics of spinodal decomposition without significantly influencing the incubation time for the formation of the .alpha.+.gamma. phase. Accordingly, at elevated levels of cold work, subsequent to the final low temperature age, the spinodal transformation can be made to go essentially to completion prior to the nucleation of the .alpha.+.gamma. phase resulting in materials having excellent combinations of high strength and high ductility. The level of cold work typically employed to cause this effect is of the order of 75% for a Cu-9Ni-6Sn alloy.
Unfortunately, however, it is a general rule for copper alloys, (and, indeed for all metals) that cold rolling metal strip at levels of cold work in excess of 25-35% gives rise to a phenomenon termed "fibering", or "texturing". The terms are somewhat misleading, as we shall further discuss, however, associated with this cold work texturing are differences in ductility (anisotropy in formability) depending on the test direction in the sheet. As the level of cold work exceeds 40%, serious degradation in transverse formability occurs, i.e., the ability to form the material with its bend axis parallel to the original rolling direction requires more and more generous bend radii.
This anisotropy is primarily due to grain elongation in the direction of rolling. The grain boundary area represents a plane of weakness for cracks to nucleate.
Concomitant with this grain elongation is a rotation of preferred (easy) slip plans within the grain giving rise to the development of a preferred crystallographic rolling texture which may further aggravate this transverse formability. Typically, in most copper alloys significant transverse anisotropy in formability is observed to occur when the grain size aspect ratio (ratio of the length to the width) approaches 1.3 to 1.5. As one continues to plastically deform the metal to higher levels of cold work, anisotropy rapidly increases. At 75% cold work, one may observe in excess of an order of magnitude difference in formability depending on the direction of test within the sheet.
Typically, for copper-based alloys, the "brass" texture develops at these elevated levels of cold work. This texture is characterized as a (110)&lt;112&gt; texture in which a preponderance of (110) planes are parallel to the rolling plane and they, in turn, are orientated such that their &lt;112&gt; direction is parallel to the sheet rolling direction.
It can therefore be seen that, elevated levels of cold work have been required to accelerate the spinodal transformation and develop high strength. The materials so processed exhibit excellent ductility either in wire form or in sheet longitudinally, but exhibit very poor transverse sheet ductility due primarily to both grain elongation effects and to the brass texture development inherent at these levels of cold work.
To avoid this phenomenon in strip, one must reduce the level of cold work prior to final aging while still attaining the essentially complete spinodal transformation required to attain high strength. This has been achieved, for example, in the Cu-Ni-Sn alloys containing prescribed amounts of Mo, Ta, Va or Fe included therein (U.S. Pat. No. 4,260,432). Commercially, however, these alloys cannot be readily thin slab cast in air due to the very high reactivity of these quaternary additives with oxygen which tend to react rapidly and slag to the surface during melting. This adversely effects the mechanical properties of the alloy. In order to prevent this, processing under a static vacuum or deoxygenated system would be necessary. Consequently, there is still a need in the art to achieve a high strength spinodal Cu alloy which is isotropically ductile and formable in strip form, and which can be made by typical air melting techniques.
In summary, in order to develop high strength isotropic material, apparently, two possible directions exist:
(1) To accelerate the spinodal decomposition process allowing it to develop to a further extent prior to nucleation of the discontinuous embrittling grain boundary reaction.
(2) To inhibit nucleation of the discontinuous reaction.
In either case, this must be accomplished without the grain elongation and brass texturing associated with high levels of cold work prior to final low temperature aging sequence.
Fourth element additions made to the Cu-Ni-Sn system in an attempt to effect (1) resulted in a spinodal demiscibility that was either unaffected or negatively affected (i.e., the transformation kinetics were retarded).
Detailed examination (transmission electron microscopy) at the onset of the nucleation of the discontinuous .alpha.+.gamma. transformation suggested that this process appears to occur at preferred crystallographic grain boundary sites. This observation is entirely consistent, thermodynamically. In principle, if the statistical number of these preferred sites could be reduced, nucleation should be retarded.
I have now discovered that by inducing a preferred enhanced recrystallization texture (as hereinafter defined) in the alloy prior to final aging, one can significantly suppress the onset of the (.alpha.+.gamma.) embrittling reaction at the grain boundaries (i.e., the nucleation time for sigmoidal onset occurs at significantly longer aging times). Since the kinetics of the demiscibility process are insensitive to crystallographic orientation this process is unaffected and proceeds normally.
Accordingly, the spinodal transformation can proceed to a further extent, i.e., higher strengths can be achieved prior to the onset of embrittlement. Surprisingly, this can be achieved at low levels of final cold work (0-35%) before aging while still attaining (after low temperature aging) essentially complete spinodal decomposition. These levels of cold work are sufficiently low enough such that negligible anisotropy in formability is observed. Since highly reactive additions are not required, the alloy can be commercially air processed.
It should be emphasized, that this discovery is in opposition to what one would expect based upon prior art teaching. In general, commercial practice dictates an extended high temperature annealing cycle to promote a random recrystallization texture. In this teaching, the presence of a metastable recrystallization texture is the key in effecting the retardation of the embrittling reaction, and hence a high strength, highly ductile isotropic material is attained.